Polyolefins such as polyethylene and polypropylene may be prepared by particle form polymerization, such as slurry polymerization or gas phase polymerization.
The reaction of polymerization of ethylene is highly exothermic, it releases about 945 kWh per ton of polyethylene produced. Furthermore, the manufacturing units of polyethylene are major consumers of energy. Large amounts of heat from the industry are discharged into the atmosphere, which not only causes loss of energy but also adds to air pollution.
Other equipment and operations within a polyolefin manufacturing process consume energy. Noteworthy consumers of electricity within a polyolefin plant, for example, may include the pumps that circulate the liquid reaction mixture in the polymerization reactors (e.g., loop slurry reactors), the pumps that circulate the cooling medium (e.g., treated water) through the polymerization reactor jackets, the compressors that pressurize and return recycled diluent (and/or monomer) to the polymerization reactor, the blowers used to convey fluff and pellets, and the extruders that convert the polyolefin fluff to polyolefin pellets. Significant users of steam in a typical polyolefin plant may include heaters that flash liquid in the effluent of the polymerization reactor, and fractionation columns that process recovered diluent and/or monomer. In particular, the demand for steam is approximately 300 kWh per ton of polyethylene produced. Relatively large consumers of fuel gas may include activation processes (which may utilize high heat) of the polymerization catalyst, and operations that maintain adequate combustible content in the plant flare header (in the feed to the flare). In general, extensive energy is required to polymerize the monomer and comonomer to polyolefin fluff, to process recycled effluent from the reactor, and to convert the polyolefin fluff to pellets.
Therefore, the production of polyolefin is an energy-intensive process, consuming electricity, steam, fuel gas, and so on. Such energy consumption generally contributes significant cost to the production of polyolefins, as well as to the downstream products of polyolefins distributed to the customer.
WO 2009010514 describes a process for energy optimization of energy consumption in a polymerization process and to a polymerization unit. Herein the Coefficient of Performance used is the ratio of the quantity of heat produced to the electrical energy consumed, as described in the examples. All of the energy recovered from the cooling water is reused at a higher temperature, but at a cost of a lot of added mechanical/electrical energy. Herein, a gas compressor is required, which is very energy consuming.
Thus, there is still a need to improve the energy efficiency of polyolefin production processes. It is accordingly one of the objects of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.